Surface coatings can provide medical articles, such as those that are implanted or temporarily inserted into the body, with a variety of distinct benefits. These benefits include lubricity and wettability, passivity against protein absorption, antimicrobial properties, drug delivery, biocompatibility and hemocompatibility. The demand for medical articles having these types of coatings is rapidly increasing because they generally improve the function of the device upon implantation or insertion in the body. However, while these properties can provide clear advantages for the function of these devices, the preparation of these coatings can, in many cases, be technically challenging and also quite costly.
Medical articles are typically prepared from plastic or metal biomaterials, or combinations of these biomaterials. Generally, plastic medical articles provide good substrates for the bonding and immobilization of coating materials, as the plastic surface can be reacted with chemical groups that are provided with the coating material. On the other hand, the immobilization of coating materials on metal substrates is generally more challenging because, in many cases, the metal surface is not able to directly covalently bond the reactive group. To overcome this, a base layer of material, often called a “priming layer” or a “tie layer”, is disposed on the surface to provide a material to which a subsequent coating material can react. Therefore, many metal-containing medical articles having coatings include two or more coated layers, at least one of which is a base layer that facilitates the immobilization of materials of a second layer.
To maintain the integrity of the coating, the material of the base layer should remain continuously contacted with the metal surface of the device after the coating is formed and during use of the coated device. Problems with the coating may be seen if a portion of the coated base layer separates from the surface, which can result in delamination of all or portions of the coated materials from the surface of the device. As a result, surface properties may be lost before or during use, for example, before or during implantation or insertion into the body.
For some medical articles which are flexed or bent during use, the material of the base layer should be compliant. A compliant base layer can prevent the coating from cracking or delaminating.
Parylene™ (poly(para-xylylene) is commonly used as a base layer material. Parylene™ base layers are typically very thin (0.1 micron to 75 microns), continuous, inert, transparent, and conformal films. Parylene™ is applied to substrates in an evacuated deposition chamber by a process known as vapor deposition polymerization (VDP). This involves the spontaneous resublimation of a vapor that has been formed by heating di-para-xylylene, which is a white crystalline powder, at approximately 150° C., in a first reaction zone. The vapor resulting from this preliminary heating is then cleaved molecularly, or pyrolyzed, in a second zone at 650° C. to 700° C. to form para-xylylene, a very reactive monomer gas. This monomer gas is introduced to the deposition chamber, where it resublimates and polymerizes on substrates at room temperature and forms a transparent film. In the final stage, para-xylylene polymerizes spontaneously onto the surface of objects being coated. The coating grows as a conformal film (poly-para-xylylene) on all exposed substrate surfaces, edges and in crevices, at a predictable rate. Parylene™ formation is spontaneous, and no catalyst is necessary.
While the benefits of a Parylene™ base layer can be clearly seen, there are various drawbacks to using this process in coating processes for metal medical articles wherein a base or tie layer is needed to form a coating. For example, as indicated above, the process of Parylene™ deposition is rather involved and furthermore requires the use of costly apparatus to carry out the vapor deposition process. Also, in order to ensure that an adequate Parylene™ layer is formed on the surface of the device substrate, it is typically necessary to thoroughly remove oils and contaminants from the device surface. This can add time to the coating process and also subjects the coated article to potential defects in the coating if it not cleaned adequately. Furthermore, in order to promote sufficient adhesion between the device surface and Parylene™ layer, the surface of the metal article typically needs to be pretreated with a silane material. This, again, can add time and expense to the coating process. Another approach is to apply fluorinated materials such as Teflon™ to the metal surface. These coatings, however, can be excessively thick, have relatively low adhesion and elasticity, and can crack under stress.
Another challenge for providing coatings relates to those implantable devices that are more complex in terms of geometry. For example, small implantable medical devices, such as stents, often have intricate geometries. In some cases, when these medical devices having intricate geometries are subjected to a coating procedure, webbing or bridging of the coating solution may occur, resulting in a coating that hinders the device from functioning properly. Other coating reagents and techniques utilize light to fix the coating compound on the device surface. However, methods involving light activation can potentially be inadequate for providing uniform coatings over the entire surface of the device. In particular, inner surfaces of devices can be difficult to access with an activating amount of light.